1. Field of the Invention
This invention relates to detector elements for infrared (IR) imaging arrays, and more particularly to a microbolometer detector element with enhanced sensitivity.
2. Description of the Related Art
Microminiature bolometers (microbolometers), such as those described in U.S. Pat. No. 5,286,976, entitled "MICROSTRUCTURE DESIGN FOR HIGH IR SENSITIVITY", issued Feb. 15, 1994 to Barrett E. Cole, and in U.S. Pat. No. 5,300,915, entitled "THERMAL SENSOR", issued Apr. 5, 1994 to Robert E. Higashi et al., are used as detector pixel elements in two-dimensional IR imaging arrays.
A microbolometer generally consists of a polycrystalline semiconductor layer whose electrical resistivity varies as a function of its temperature. The layer material is chosen so that it absorbs optical radiation over a design wavelength range, which is generally in the IR region of the spectrum. The semiconductor layer is fabricated on a silicon substrate, which also contains integrated readout circuitry for monitoring the layer's resistivity. An array of microbolometers are fabricated on a single substrate to create a two-dimensional imaging array.
In operation, incident IR radiation is absorbed by the semiconductor layer, causing a change in the layer's temperature. The temperature change causes a corresponding change in the layer's resistivity, which is monitored by the readout circuitry.
The microbolometers described above utilize a continuous semiconductor absorptive layer disposed on a dielectric "bridge" structure that has been fabricated on a silicon substrate. The bridge structure supports the layer so that it is spaced away from the silicon substrate surface. Incident optical radiation is partially absorbed by the layer. A portion of the non-absorbed light passes through the layer and strikes a reflective metal layer that has been fabricated on the silicon substrate surface directly below the layer (under the "bridge" structure). The metal layer reflects the radiation back to the absorptive layer, where it again gets partially absorbed. By proper adjustment of the spacing between the absorptive layer and the reflective metal layer, constructive optical interference can be achieved between the incident optical radiation and the reflected optical radiation at the absorptive layer. However, for any given absorptive layer to metal layer spacing the constructive interference only occurs for wavelengths in which the antinodes of the incident and reflected radiation components overlap. For these wavelengths, the electromagnetic field strength at the absorptive layer will be increased, resulting in higher absorption.
The microbolometer's sensitivity is dependent on the semiconductor layer's absorption coefficient (.alpha.), the temperature change caused in the layer per unit of optical energy absorbed (dT/dA) and the electrical resistance change in the layer caused per unit of temperature change (dR/dT). DT/dA varies inversely with the heat capacity of the semiconductor layer. Therefore, as the mass of the semiconductor layer decreases, dT/dA increases. The absorption enhancement caused by constructive interference effects require that any incident optical radiation that is not absorbed by the layer be transmitted to the metal layer below. If the absorptive layer reflects any optical radiation, total absorption is reduced. Reflectance at the absorptive layer can be reduced by precisely controlling the uniformity of the layer, which is difficult to accomplish with current microbolometer processing techniques.